From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id F199BCAC5A7 for ; Thu, 25 Sep 2025 09:48:00 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1v1iYW-0001K1-7q; Thu, 25 Sep 2025 05:46:38 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1v1iYS-0001Hm-4S for qemu-rust@nongnu.org; Thu, 25 Sep 2025 05:46:24 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1v1iXv-0001bd-Sc for qemu-rust@nongnu.org; Thu, 25 Sep 2025 05:46:20 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1758793548; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=XvkLe5PlKeo0HdSyG7FFTgKImW+3jifmX4pZMoyAdkQ=; b=E1BK27jJ/OV4f3DUwkWRI00/Oy9RwF5cATm1ayWSnh5PUmzlGwyRX5o24tAgc8Y4dLNO5u ngZalrGdrnMg/sgvFjreVbWQmjj69RwqD05f2yehSmOkQs7a4s9BioRXV4+PvU//NAn42C UXdOiFnzF4q8ZJ5di2Bk3EJ2RaxmOX8= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-126-tUPuQASIN2CtD6daej_t6g-1; Thu, 25 Sep 2025 05:45:43 -0400 X-MC-Unique: tUPuQASIN2CtD6daej_t6g-1 X-Mimecast-MFC-AGG-ID: tUPuQASIN2CtD6daej_t6g_1758793542 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id ECA18195605B; Thu, 25 Sep 2025 09:45:41 +0000 (UTC) Received: from toolbx.redhat.com (unknown [10.42.28.163]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id ACEE830002D1; Thu, 25 Sep 2025 09:45:36 +0000 (UTC) From: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= To: qemu-devel@nongnu.org Cc: Hanna Reitz , =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= , =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= , Kevin Wolf , qemu-rust@nongnu.org, Richard Henderson , Markus Armbruster , Christian Schoenebeck , =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , qemu-block@nongnu.org, Stefan Weil , "Dr. David Alan Gilbert" , Manos Pitsidianakis , Gerd Hoffmann , devel@lists.libvirt.org, Paolo Bonzini Subject: [PATCH v4 09/23] util: introduce some API docs for logging APIs Date: Thu, 25 Sep 2025 10:44:27 +0100 Message-ID: <20250925094441.1651372-10-berrange@redhat.com> In-Reply-To: <20250925094441.1651372-1-berrange@redhat.com> References: <20250925094441.1651372-1-berrange@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.444, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, T_SPF_HELO_TEMPERROR=0.01, T_SPF_TEMPERROR=0.01 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-rust@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: QEMU Rust-related patches and discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-rust-bounces+qemu-rust=archiver.kernel.org@nongnu.org Sender: qemu-rust-bounces+qemu-rust=archiver.kernel.org@nongnu.org This addresses two key gotchas with the logging APIs: * Safely outputting a single line of text using multiple qemu_log() calls requires use of the qemu_log_trylock/unlock functions to avoid interleaving between threads * Directly outputting to the FILE object returned by qemu_log_trylock() must be discouraged because it prevents the inclusion of configured log message prefixes. Reported-by: Markus Armbruster Signed-off-by: Daniel P. Berrangé --- include/qemu/log-for-trace.h | 35 ++++++++++++++++++++++++++++++++++- include/qemu/log.h | 26 ++++++++++++++++++++++++++ rust/util/src/log.rs | 7 +++++++ 3 files changed, 67 insertions(+), 1 deletion(-) diff --git a/include/qemu/log-for-trace.h b/include/qemu/log-for-trace.h index d47c9cd446..4e05b2e26f 100644 --- a/include/qemu/log-for-trace.h +++ b/include/qemu/log-for-trace.h @@ -29,7 +29,40 @@ static inline bool qemu_loglevel_mask(int mask) return (qemu_loglevel & mask) != 0; } -/* main logging function */ +/** + * qemu_log: report a log message + * @fmt: the format string for the message + * @...: the format string arguments + * + * This will emit a log message to the current output stream. + * + * The @fmt string should normally represent a complete line + * of text, ending with a newline character. + * + * If intending to call this function multiple times to + * incrementally construct a line of text, locking must + * be used to ensure that output from different threads + * is not interleaved. + * + * This is achieved by calling qemu_log_trylock() before + * starting the log line; calling qemu_log() multiple + * times with the last call having a newline at the end + * of @fmt; finishing with a call to qemu_log_unlock(). + * + * The FILE object returned by qemu_log_trylock() does + * not need to be used for outputting text directly, + * it is merely used to associate the lock. + * + * FILE *f = qemu_log_trylock() + * + * qemu_log("Something"); + * qemu_log("Something"); + * qemu_log("Something"); + * qemu_log("The end\n"); + * + * qemu_log_unlock(f); + * + */ void G_GNUC_PRINTF(1, 2) qemu_log(const char *fmt, ...); #endif diff --git a/include/qemu/log.h b/include/qemu/log.h index aae72985f0..867fe327e4 100644 --- a/include/qemu/log.h +++ b/include/qemu/log.h @@ -41,7 +41,33 @@ bool qemu_log_separate(void); /* Lock/unlock output. */ +/** + * Acquires a lock on the current log output stream. + * The returned FILE object must be passed to + * qemu_log_unlock() to later release the lock. + * + * This should be used to protect a sequence of calls + * to qemu_log(), if they are being used to incrementally + * output a single line of text. For qemu_log() calls which + * output a complete line of text it is not required to + * take explicit locks. + * + * The returned FILE object may be used to directly + * output log messages, however, doing so will prevent + * the inclusion of configured log message prefixes. + * It is thus recommended that this be used sparingly, + * only in cases where it is required to dump large + * data volumes. Use of qemu_log() is preferred for + * most output tasks. + * + * Returns: the current FILE if available, NULL on error + */ FILE *qemu_log_trylock(void) G_GNUC_WARN_UNUSED_RESULT; + +/** + * Releases the lock on the log output, previously + * acquired by qemu_log_trylock(). + */ void qemu_log_unlock(FILE *fd); /* Logging functions: */ diff --git a/rust/util/src/log.rs b/rust/util/src/log.rs index af9a3e9123..eaf493f0df 100644 --- a/rust/util/src/log.rs +++ b/rust/util/src/log.rs @@ -55,6 +55,13 @@ impl LogGuard { /// writeln!(log, "test"); /// } /// ``` + /// + /// Note that directly writing to the log output will prevent the + /// inclusion of configured log prefixes. It is thus recommended + /// that this be used sparingly, only in cases where it is required + /// to dump large data volumes. Use of [`log_mask_ln!()`](crate::log_mask_ln) + /// macro() is preferred for most output tasks. + pub fn new() -> Option { let f = unsafe { bindings::qemu_log_trylock() }.cast(); NonNull::new(f).map(Self) -- 2.50.1